The present invention generally relates to hydraulic boosters for amplifying an input operational force by using a hydraulic power and more particularly, to a hydraulic booster which is suitable for obtaining amplified output of a hydraulic master cylinder for generating a braking force of a motor vehicle.
Conventionally, in valve portion of a hydraulic booster used in a master cylinder of a motor vehicle, relative position of an input rod and a power piston is detected such that the valve is opened or closed in accordance with the relative position of the input rod and the power piston. Namely, when the power piston is retracted relative to the input rod, a power source pressure for amplification is introduced into a boost chamber. On the other hand, when the power piston is advanced relative to the input rod, the boost chamber is communicated with a reservoir such that the boost chamber is depressurized. Meanwhile, generally, a small neutral zone is provided for controlling rise and fall of pressure in the boost chamber. Thus, when the power piston is disposed at a neutral position relative to the input rod, a pressure in the boost chamber, i.e. a boost pressure is maintained at a constant value.
In order to perform the above described control in the known hydraulic booster, it is necessary either to provide the valve portion in the power piston such that the valve portion is displaced together with the power piston and the input rod, or, to detect the above described relative displacement between the power piston and the input rod such that the detected result is transmitted to the valve portion provided at another fixed location.
In the above described two methods, a transmission mechanism (link mechanism) is indispensable in the latter method, so that the latter method becomes complicated and expensive. Hence, in most cases, the former method is employed. However, in the former method, the power source pressure is required to be introduced into the power piston. To this end, it is necessary to employ either a first method in which a valve chamber in the power piston and a supply passage of the power source pressure are communicated with each other by a flexible hose or a second method in which an annular intermediate inlet chamber is formed on an outer periphery of the power piston so as to be slidably isolated by two high-pressure seals and is communicated with the valve chamber in the power piston by a hole formed on the power piston. However, the flexible hose of the first method poses various problems such as large setting space, low resistance against vibration, short service life, etc. and thus, is not suitable for practical use. Therefore, the second method, i.e. the intermediate inlet chamber is generally employed.
One concrete example of a prior art arrangement employing this second method is shown in FIG. 1. In FIG. 1, a power piston 1 is axially movably fitted into a bore of a booster body 2. An input rod 3 is provided rearwards of the power piston 1. A spool valve 4 is axially slidably fitted into a valve chamber in the power piston 1. A boost chamber 5 is formed for applying a boost pressure to a portion of a rear face of a stepped portion of the power piston 1. An intermediate inlet chamber 6 for introducing a power source pressure thereinto is provided between an inner face of the bore of the booster body 2 and an outer periphery of the power piston 1 and is communicated with an output circuit of a pump 11. High-pressure seals 7 and 8 are provided for slidably sealing right and left opposite ends of the intermediate inlet chamber 6. This prior art arrangement further includes a return spring 9 for returning the power piston 1 to its original position, a return spring 10 for returning the spool valve 4 to its original position, a reservoir 12 and an accumulator 13.
When the spool valve 4 is not present in the valve chamber in the power piston 1, the valve chamber is, respectively, communicated, via holes 14, 15 and 16 formed on the power piston 1, with the intermediate inlet chamber 6, the boost chamber 5 and a depressurizing path 17 for the boost chamber 5, which leads to the reservoir 12. The spool valve 4 forms a valve portion between the holes 14 and 16 so as to open and close a hydraulic path between the intermediate inlet chamber 6 and the boost chamber 5 and a hydraulic path between the boost chamber 5 and the depressurizing path 17. Namely, when an operational input is zero and a passage 18 formed in the spool valve 4 coincides, in position, with the hole 16, the spool valve 4 cuts off communication between the holes 14 and 15 formed through the valve chamber so as to release pressure in the boost chamber 5.
On the other hand, when the input rod 3 has been depressed, relative displacement of the input rod 3 and the power piston 1 is transmitted to the spool valve 4. Thus, the spool valve 4 is displaced in the leftward direction in FIG. 1 so as to cut off communication between the hole 16 and the passage 18 initially. Thereafter, with a slight delay, the holes 14 and 15 are communicated with each other. Hence, the power pressure transmitted from the intermediate inlet chamber 6 is introduced into the boost chamber 5 so as to depress the power piston 1. Therefore, in this prior art arrangement, a piston 19 for generating hydraulic pressure is advanced such that a hydraulic pressure obtained by amplifying the operational input of the input rod 3 in proportion thereto is outputted from a cylinder 20. Meanwhile, reference numeral 21 denotes a liquid replenishment chamber for the cylinder 20 communicating with the reservoir 12.
The conventional hydraulic booster referred to above has such a drawback that since the intermediate inlet chamber 6 is directly connected with the power source, the seals 7 and 8 are subjected to the high power source pressure at all times and thus, sliding resistance forces of the seals 7 and 8 become excessively large.
Meanwhile, due to the above described fact, an urging force of the return spring 9 also should be large. Namely, in apparatuses of this kind, the return spring is required to return the advanced piston to its original position at the time of release of the operational force. The urging force of the return spring should be larger than a total of sliding resistance forces of the seals 7 and 8 and a seal for the piston 19. However, if the sliding resistance forces of the seals 7 and 8 are large, the urging force of the return spring 9 should also be large accordingly. Therefore, in the prior art hydraulic booster, a resistance force against advance of the piston amounts to an addition of the high sliding resistance forces of the seals and the large urging force of the return spring 9. Until a counterforce produced by the pressure of the boost chamber 5 exceeds the resistance force against advance of the piston, the power piston 1 cannot advance. Accordingly, the prior art hydraulic booster has such inconveniences that the start of production of the output pressure in the chamber 20 is delayed especially at an initial stage of operation and the pressure in the boost chamber 5 becomes much higher than the pressure produced in the chamber 20.